Download the correct “minecraftforge-installer-x.x.x-x.x.x.xxx.jar” for a easy Forge installation.

Run the “minecraftforge-installer-x.x.x-x.x.x.xxx.jar“, select “Install Server“, select a folder where the server will be installed (eg. “C:\Users\<user>\AppData\Roaming\.minecraft_server“) and click on “Done“.

Go to the selected folder and open “minecraftforge-universal-x.x.x-x.x.x.xxx.jar“.

Open “minecraft_server.x.x.x.jar” with WinRar or 7zip.

Drop everything inside the “minecraftforge-universal-x.x.x-x.x.x.xxx.jar” into the “minecraft_server.x.x.x.jar“.